1. Field of the Invention
The invention exists in the technical field of a crystal oscillator having frequency temperature characteristics depending on a crystal unit, and in particular relates to a crystal oscillator whose frequency temperature characteristics are corrected within a standard.
2. Description of the Related Art
Since a crystal oscillator has a remarkably higher Q value in comparison with an oscillator using ceramic, etc., and has excellent frequency stability, the crystal oscillator may be incorporated in various types of electronic devices using frequency and time as a reference source. As one type of crystal oscillator, there is a crystal oscillator in which a crystal unit having AT cut as the representative cutting angle and having a frequency band of approximately 10 through 100 MHz is used.
FIGS. 5, 6A and 6B are views describing a related art crystal oscillator, in which FIG. 5 is a circuit diagram of the related art crystal oscillator, and FIGS. 6A and 6B are frequency temperature characteristics views of the related art crystal oscillator.
A crystal oscillator forms a resonance circuit using a voltage dividing capacitor (Ca and Cb) and a crystal unit 1 as an inductor component. The crystal oscillator is made into a so-called Colpitts type oscillator in which an oscillation frequency by the resonance circuit is feedback-amplified by an oscillation amplifier 2. In this example, the oscillation amplifier 2 is made into a common collector as a transistor Tr. A voltage-variable capacitive element (variable capacitance diode) 3 is connected to the crystal unit 1 in series, and a control voltage Vc is applied between terminals of the voltage-variable capacitive element 3.
The control voltage Vc, which is, for example, automatic frequency control voltage (AFC voltage), is input from an AFC circuit incorporated on a set substrate (not illustrated) of an electronic device along with a crystal oscillator. Since the AFC voltage differs in accordance with specifications of an electronic device, the AFC voltage is normally divided and applied by the first resistor Ra and the second resistor Rb. Therefore, by selecting a portion in which the capacitance change of the voltage-variable capacitive element 3 with respect to voltage is linear, the frequency change characteristics of the oscillation frequency are improved. Incidentally, symbols R1, R2 and R3 in the drawings denote bias resistors, RC denotes a high frequency blocking resistor, Ct denotes a frequency-adjusting capacitor, Vcc denotes a power source, and Vout denotes output.
In such a crystal oscillator, the oscillation frequency changes depending on a temperature, particularly depending on the frequency temperature characteristics of a crystal unit (AT cut) 1. In the AT cut crystal unit, the frequency temperature characteristics are made into a cubic curve having an inflection point in the vicinity of normal temperature (approximately 25° C.). In the AT cut crystal unit, a cubic curve (curve A of FIG. 6A) having the maximum value and the minimum value at both sides of the standard temperature range (−20 through 70° C.) and a cubic curve (curve B of FIG. 6B) having the maximum value and the minimum value at a normal temperature side are brought about in accordance with minutely different cutting angles.
And, for example, if the operational temperature of the crystal unit 1 is a constant-temperature type in which the operational temperature of the crystal unit 1 is fixed by a temperature control circuit having a heater (not illustrated), etc., and is stabilized to be high, frequency temperature characteristics (curve A of FIG. 6A) having the maximum value at a high temperature side are selected. In this case, if the operational temperature is set to a normal temperature or less, the temperature cannot be lowered when the temperature exceeds the normal temperature. Therefore, the operational temperature is set to the maximum value over the normal temperature. At the maximum value, a fluctuation width (the amount of change) of the oscillation frequency centering around the temperature is decreased.
Further, in a typical crystal oscillator, since the fluctuation width of the oscillation frequency is suppressed even if the temperature changes from the normal temperature to a low temperature side or a high temperature side, the frequency temperature characteristics (curve B of FIG. 6B) having the maximum value and the minimum value are selected as in the above-described case.
Incidentally, JP-UM-A-59-118307, JP-UM-A-61-81208 and JP-A-6-85538 each discloses a related art crystal oscillator.
However, in the crystal oscillator having the above-described configuration, the frequency temperature characteristics of the crystal oscillator depend on the crystal unit 1, and the frequency temperature characteristics of the crystal unit 1 depend particularly on a delicate cutting angle (in the unit of second). Therefore, strict work is required for cutting the crystal unit 1 (artificial crystal). In addition, if the cutting angle deviates from the standard cutting angle or if influence of the temperature characteristics by other circuit elements is great, the frequency deviation with respect to a temperature becomes below the standard. Thus, there is a problem by which the productivity of crystal oscillators is lowered.
Accordingly, for example, it is considered that a capacitor, which has temperature characteristics, and a capacitance value of which has a positive or negative characteristic, is applied as a capacitor Ct for adjusting the oscillation frequency. That is, the frequency temperature characteristics are brought into the standard by turning the frequency temperature characteristics centering around the normal temperature (i.e., approximately 25° C. of the inflection point) by the temperature characteristics of the capacitor. In summary, the series equivalence capacitance (load capacitance) observed from both ends of the crystal unit 1 is varied by the capacitance of the capacitor changing in accordance with a temperature, and the frequency temperature characteristics are corrected.
For example, for a case where the maximum value or the minimum value of the frequency temperature characteristics at the standard temperature range (−20 through 70° C.) exceeds an allowable deviation ±α ppm as at the curve A in FIG. 6A, a capacitor having a negative characteristic, which slopes down rightward (i.e., which capacitance value is decreased in line with a temperature rise), is applied to the curve A. Therefore, since the capacitance value of the capacitor is decreased in line with a temperature rise to cause the oscillation frequency to increase, the frequency temperature characteristics turn left to cause both the maximum value and the minimum value to be brought into the standard (curve A′). Further, in this case, the standard can be satisfied with frequency temperature characteristics, which is asymmetrical and only the maximum value thereof is outside the standard.
In addition, for a case where the maximum value and the minimum value of the frequency temperature characteristics are brought out of the frequency deviation at both sides inside the standard temperature even the maximum value and the minimum value thereof are within the frequency deviation ±α ppm as at the curve B of FIG. 6B, a capacitor having a positive characteristic, which slopes up rightward (i.e., which capacitance value is increased in line with a temperature rise), is applied. Therefore, since the capacitance value of the capacitor is increased in line with a temperature rise to cause the oscillation frequency to decrease, the frequency temperature characteristics turn right, and are brought into the frequency deviation even at both sides of the standard temperature (curve B′ of FIG. 6B). In addition, the standard can be satisfied with the frequency temperature characteristics, in which only the high temperature side thereof is outside the standard.
However, in fact, only a few types of capacitors have temperature characteristics, and most of the capacitors have negative characteristics which slope down rightward. Therefore, in these cases, even it is possible to correct the frequency temperature characteristics by turning right, it is difficult to correct the frequency temperature characteristics by turning left. Further, in either case, for example, only several types (i.e., four types) of capacitors having negative characteristic are available, there are large variation in characteristics among them, and thus it becomes difficult to design and fabricate the crystal oscillator. In particular, in the case of a constant-temperature type crystal oscillator whose frequency deviation is, for example, on the order of ppb ( 1/10 billion), there is a problem by which delicate adjustment becomes difficult.